For heat treatment of metal, various heat treatments have conventionally been used depending on application purposes, the heat treatments including a standardization treatment such as annealing/normalizing, a hardening/toughening treatment, such as quenching/tempering and thermal refining, and a surface hardening treatment, such as nitriding and surface improvement. While these atmosphere heat treatments are performed in atmosphere gases, such as atmospheric air, inert gases, oxidizing gases, and reducing gases, which are supplied to a heat treatment furnace, the properties of metals that are subjected to the heat treatments are drastically changed by components of these atmosphere gases. Accordingly, it is necessary to control the components of the atmosphere gases supplied into the heat treatment furnace with sufficient precision and to visualize the status of the atmosphere in the furnace with high precision.
As a first conventional technology that performs feedback control on the flow rate of the gas supplied to a heat treatment furnace in response to a signal coming from an oxygen potentiometer placed inside the heat treatment furnace, a method of adjusting the atmosphere gas in a bright annealing furnace disclosed in Patent Literature 1 (Japanese Patent Laid-Open No. 3-2317) will be described with reference to FIG. 1. In FIG. 1, exothermic converted gas is supplied from an exothermic converted gas generator 11 to a gas mixer 13 via a dehumidifier 12, while hydrocarbon gas is supplied from a hydrocarbon gas feeder 14 to the gas mixer 13 via a flow control valve V1 so that the hydrocarbon gas is mixed with the exothermic converted gas.
The mixed gas is heated and combusted at high temperature (1100° C.) in a gas converter with heating function 15, and then the gas is quenched and dehumidified in a gas quenching/dehumidifier system 16, before being supplied to a bright annealing furnace 17. Oxygen partial pressure is measured by the oxygen potentiometer 18 provided inside the bright annealing furnace 17, and based on this measurement value, carbon potential (CP) is calculated by a carbon potential computation controller 19. Then, the calculated value is compared with a preset carbon content in an object to be treated, and the flow rate of hydrocarbon gas supplied to the gas mixer 13 is feedback-controlled via the flow control valve V1 so that the calculated value is matched with the preset carbon content. This prevents oxidation and decarbonization of the material to be treated in the bright annealing furnace 17.
Next, as a second conventional technology, a method of controlling furnace gas in bright heat treatment disclosed in Patent Literature 2 (Japanese Patent Laid-Open No. 60-215717) will be described with reference to FIG. 2.
In FIG. 2, an oxygen analyzer 22 detects the partial pressure of residual oxygen in a heat chamber 21. When the detection value is higher than a set value set in an oxygen partial pressure setting unit 24, hydrocarbon gas and reducing gas are supplied to the heat chamber 21, whereas when the detection value is lower than the set value, oxidizing gas such as air is supplied to the heat chamber 21 so as to control the amount of residual oxygen to be constant.
A carbon monoxide analyzer 23 also detects the partial pressure of residual carbon monoxide in the heat chamber 21, and when the detection value is higher than a set value set in a carbon monoxide partial pressure setting unit 25, inert gas, such as nitrogen, is discharged to the outside of the furnace while being supplied to the heat chamber 21, so that the amount of residual carbon monoxide is controlled to be constant. As a consequence, even when moisture, oxides, and oil and fat adhere to the surface of metals to be treated, the bright treatment is implemented without causing oxidation, decarbonization, carbon deposition, and carburization.
Now, as a third conventional technology, a method and an apparatus for heat treatment disclosed in Patent Literature 3 (U.S. Pat. No. 4,521,257) will be described with reference to FIG. 3.
In FIG. 3, a regulator 38 calculates CP in each of a carburization chamber 35, a diffusion chamber 36, and a soaking chamber 37 based on detection values of oxygen sensors 32, 33, and 34. The calculated values and respective CP set values are compared, and openings of respective flow rate valves are adjusted so as to control each supply flow rate of enrich gas supplied to each of the chambers.
Moreover, there is provided a sequencer 39 that controls the process in a carburizing treatment device, the sequencer 39 being configured to execute a command to cause the regulator 38 to stop and/or resume PID adjustment in accordance with the status of the carburizing device. Accordingly, during a heat treatment period including the time of opening an opening of the furnace, the CP is controlled to be constant.
Next, as a fourth conventional technology, a method and an apparatus for preventing coloration of a plate passing through reducing atmosphere furnace disclosed in Patent Literature 4 (Japanese Patent Laid-Open No. 11-80831) will be described with reference to FIG. 4.
In FIG. 4, the bright treatment is performed on a stainless steel 41 in a bright annealing furnace 42 having a color difference meter 45 provided on an outlet side. A control device 46 adjusts the circulating amount in a refining device 44 and the concentration of H2 supplied from a reducing gas supply device 43, so that a difference signal between an output signal of the color difference meter 45 and a reference signal falls within a control range. This makes it possible to manufacture uniform metallic materials with a stable coloration state.
As a fifth conventional technology, a method of calculating heat treatment conditions by using an Ellingham diagram to reduce metal oxide to metal is disclosed in Patent Literature 5 (WO 2007/061012).